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Prediction of Groundwater Nitrate Contamination after Closure of an Unlined Sheep Feedlot

^a U.S. Geological Survey, 333 West Nye Lane, Carson City, NV 89706 USA
^b Wairakei Research Centre, Institute of Geological and Nuclear Sciences Ltd., Private Bag 2000, Taupo, New Zealand
^c Horticulture and Food Research Institute of New Zealand Ltd., Palmerston North Research Centre, Private Bag 11030, Palmerston North, New Zealand
^d Hawkes Bay Regional Council, Private Bag 6006, Napier, New Zealand

View larger version (33K): [in a new window]   Fig. 1. Maraekakaho area showing the former feedlot site and sampling sites.

View larger version (121K): [in a new window]   Fig. 2. General land use types in the area. Area C includes both the feedlot land use and the current vineyard land use and so appears differently than in Fig. 1.

View larger version (23K): [in a new window]   Fig. 3. Alkalinity contour maps for June 1994 and May 2001 sampling rounds. The concentration of the deep piezometer was used for the nested GMK94 peizometers in these maps.

View larger version (23K): [in a new window]   Fig. 4. Nitrate-N contour maps for June 1994 and May 2001 sampling rounds. The average concentration of all three piezometers was used for the nested GMK94 piezometers in these maps.

View larger version (30K): [in a new window]   Fig. 5. Groundwater HCO3 trends through time from June 1994 and May 2001 for selected wells and the Maraekakaho Stream: (A) domestic wells and Maraekakaho Stream and (B) GMK94 series shallow wells.

View larger version (28K): [in a new window]   Fig. 6. Groundwater NO3–N trends through time from June 1994 and May 2001 for selected wells and the Maraekakaho Stream: (A) domestic wells and Maraekakaho Stream and (B) GMK94 series shallow wells.

View larger version (24K): [in a new window]   Fig. 7. Plot of NO3–N and alkalinity variations with depth in the aquifer for piezometer GMK94-04. Nitrate-N concentrations decrease with depth, and HCO3 increase with depth.

View larger version (24K): [in a new window]   Fig. 8. Carbon/nitrogen ratios with depth in the GMK01-SOIL1 and -SOIL2 profiles. The boxed area near the 8-m depth corresponds to low C/N ratios that represents the depth to which we estimate that excess feedlot N has penetrated.

View larger version (17K): [in a new window]   Fig. 9. Modeled NO3 concentration for selected soil depths under an active feedlot (1986–1998) that was changed to vineyard land use for the next 14 yr. The arrow indicates when the land use was changed to a vineyard. CN is concentration of N.

View larger version (18K): [in a new window]   Fig. 10. Predicted NO3 concentration for selected soil depths under a vineyard at Maraekakaho that receives an annual dressing of N fertilizer at a rate of 30 kg N ha–1. The dashed line is the New Zealand drinking water standard of 11.3 g m–3 for NO3. CN is concentration of N.